The present invention relates to phosphine based fumigation methods for controlling pests. More particularly, the present invention relates to a combination of phosphine, carbon dioxide, and elevated heat disseminated through a building interior to eradicate pests.
Fumigation is a conventional method for controlling pests in buildings, storage facilities, shipholds, trucks, factories, human habitations, or other enclosable structures. Toxic fumes or vapors are released into an enclosable structure for a limited time to exterminate rodents, insects, worms, and other destructive or infectious organisms. Conventional fumigants have included highly toxic chemicals such methyl bromide, hydrogen cyanide, ethylene dibromide, ethylene oxide, sulfur dioxide, tetrachloride, and podichlorobenzene. However, because of their extreme toxicity, bioaccumulation potential, and persistence, use of many such fumigants is now banned or tightly regulated.
Because of the hazards associated with conventional fumigants, fumigators have begun widespread use of phosphine (hydrogen phosphide) as a replacement fumigant. Phosphine leaves minimal toxic residue, does not accumulate in the biological food chain, and is relatively inexpensive. Phosphine is comparatively simple to produce at the fumigation location, typically being produced by reacting moisture with a hydrolysable metal phosphide composition such as magnesium or aluminum phosphide.
However, even phosphine fumigation is not without problems. Phosphine is itself a toxic chemical requiring care in preparation and use, and can require substantial amount of time to be ventilated from large, multileveled enclosable structures such as a building, warehouse, or storage facility. In addition, even low levels of phosphine have a detectable odor that requires time to be eliminated from the building. Conventional treatment of buildings with 500 ppm or greater phosphine concentration can require from three to ten days for treatment and aeration before the building is ready for human occupancy. While this time requirement is not of great concern for granary storage bins or the like, such substantial fumigation time greatly limits the potential for fumigation of factories or other human occupied structures.
In an effort to eliminate the use of highly toxic and bioreactive vapors, an inert gas such carbon dioxide can be employed as fumigant. For example, U.S. Pat. No. 4,989,363 to Doernemann describes a process for slowly admitting carbon dioxide in the bottom of a sealed structure while venting air originally contained in the sealed structure. Carbon dioxide is maintained in the sealed structure at a high concentration of 60% to as much as 100% for long period of time (e.g. 5-80 days), suffocating any pests in the building. While effective, this procedure has been found to be time consuming and expensive for general use.
What is needed is a fumigation method that is uses minimal amounts of toxic chemicals, is easy and inexpensive to apply, and requires a limited amount of time for action. The present invention satisfies these needs by provision of a method for fumigating an enclosed structure that includes the steps of releasing sufficient phosphine into the enclosed structure to maintain an average concentration of less than 500 ppm phosphine. In preferred embodiments less than 200 ppm phosphine is utilized, and in a most preferred embodiment between about 50 ppm and 150 ppm phosphine is released. Concurrently, carbon dioxide is released into the enclosed structure to maintain an average concentration of less than 20% carbon dioxide, preferably between about 4% and 10% carbon dioxide, and a temperature of greater than about 30 degrees Celsius, preferably between about 30 degrees Celsius to about 40 degrees Celsius, is maintained in the enclosed structure to enhance fumigation effectiveness.
Alternatively, the present invention provides for release of sufficient phosphine into the enclosed structure to maintain an average concentration of less than 500 ppm phosphine, and preferably between about 50 ppm and 150 ppm phosphine, for a time not to exceed 72 hours, and preferably about 12 to about 24 hours, while simultaneously releasing sufficient carbon dioxide into the enclosed structure to maintain an average concentration of between about 4% and 10% carbon dioxide for a time not to exceed 72 hours. Another embodiment of the invention relies on the synergistic pesticidal effect of phosphine and heat, requiring release of sufficient phosphine into the enclosed structure to maintain an average concentration of less than 200 ppm, and preferably between about 50 ppm and 150 ppm phosphine, for a time not to exceed 72 hours, while raising the temperature of the enclosed structure to above about 30 degrees Celsius, and preferably to a temperature between about 30 degrees Celsius to about 40 degrees Celsius.
In preferred embodiments, hydrolysis of a metal phosphide such as magnesium phosphide is used to release phosphine. Enough phosphine is released to maintain a desired concentration for a limited time usually not exceeding about 72 hours. In a most preferred embodiment, the time is limited to between about 24 hours to about 48 hours. With such a limited required fumigation time, it is possible to fumigate over a weekend or holidays, causing minimal interruption of work in a factory or other human occupied building.
Additional objects, features, and advantages of the present invention will be apparent upon consideration of the following detailed description and accompanying drawings.